Rapid advances are occurring in various electronic devices, especially display devices that are used for various communicational, financial, and archival purposes. For such uses as touch screen panels, electrochromic devices, light emitting diodes, field effect transistors, and liquid crystal displays, conductive films are essential and considerable efforts are being made in the industry to improve the properties of those conductive films.
There is a particular need to provide touch screen displays and devices that contain improved conductive film elements. Currently, touch screen displays use Indium Tin Oxide (ITO) coatings to create arrays of capacitive areas used to distinguish multiple point contacts. ITO coatings have significant short comings. Indium is an expensive rare earth metal and is available in limited supply from very few sources in the world. ITO conductivity is relatively low and requires short line lengths to achieve adequate response rates. Touch screens for large displays are broken up into smaller segments to reduce the conductive line length to an acceptable resistance. These smaller segments require additional driving and sensing electronics. In addition ITO is a ceramic material, is not readily bent or flexed, and requires vacuum deposition with high processing temperatures to prepare the conductive layers.
Silver is an ideal conductor having conductivity 50 to 100 times greater than ITO. Silver is used in many commercial applications and is available from numerous sources. It is highly desirable to make conductive film elements using silver as the source of conductivity, but it requires considerable development to obtain the optimal properties.
The production of touch screen sensors and other transparent conductive articles in roll-to-roll production methods on flexible and transparent substrates using “additive processes” for deposition of electrically-conductive patterns that provide the functionality of the sensor has been the subject of recent development in the industry. Of particular importance is the ability to produce a touch screen sensor that has both the desired electrical performance as well as appropriate optical properties (transmittance) in the visible portion (touch region) of the touch screen sensor. To achieve the necessary conductive and optical properties, average line widths of electrically-conductive lines in the electrically-conductive grid of less than 10 μm are greatly desired.
The flexible and transparent substrates used in such processes should be optically clear (high integrated transmittance) and colorless and exhibit low haze. The application of electrically-conductive patterns using additive processes such as flexographic printing of electrically-conductive materials or seed metal compositions requires the flexible and transparent substrate to have appropriate surface energy and roughness consistent with the scale of the fine features (for example, fine lines) to be applied. Considerable effort is being exerted in the electronics industry to achieve these necessary features.
WO 2013/063188 (Petcavich et al.) describes a method for producing a mutual capacitance touch sensor comprising a dielectric substrate by printing, using a flexographic printing process with at least a first master plate and a first ink (“printable composition”), a first pattern on a first side of a dielectric substrate; and curing the printed dielectric article. A second ink can be similarly applied and cured to form a second pattern on a second surface of the substrate. Both patterns can contain seed metal catalyst that can then be electrolessly plated with a conductive material. The resulting dielectric article is described to have a thickness of 1 μm to 1 mm and a preferred surface energy of from 20 Dynes/cm to 90 Dynes/cm. The inks used in such methods are generally non-aqueous in nature and contain various photocurable components as well as dispersed metallic particles.
It is known to use various materials to disperse metallic particles in either aqueous or non-aqueous compositions. For example, U.S. Pat. No. 8,506,849 (Li et al.) describes curable conductive inks including metallic nanoparticles and separate polymeric dispersants. Magnetic inkjet printing inks comprising dispersed magnetic nanoparticles and polymeric dispersants are described in U.S. Pat. No. 8,597,420 (Iftime et al.). In other techniques, the outer surface of metallic nanoparticles are modified to incorporate hydrophobic tails to improve dispersibility in organic solvents for inkjet printing as described for example in U.S. Patent Application Publication 2008/0090082 (Shim et al.).
There is a need for improved printable compositions (also known as inks) that contain seed metal catalyst for electroless plating. It is desired to apply (for example, printing) such improved compositions as electrically-conductive patterns of lines with a desired coloration for optical effects, stability for successful manufacturing, and electroless printing performance.
Yet, the resulting article with the electrically-conductive pattern of lines must be highly transparent and not visible when it is viewed under lighting conditions where a reflective line is visible. For this purpose, it has been determined to treat the outer surface of electrically-conductive lines (for example, composed of copper) with a blackening agent to decrease reflectivity of the metallic wires.
However, in some display devices containing capacitive touch screens in which electrically-conductive patterns are provided on both sides of a transparent substrate, the top surface of the “blackened” electrically-conductive wires can be obscured, but the bottom surface is visible and reflective through the transparent substrate.
In order to maintain thin lines in the electrically-conductive patterns, it is desirable to apply thin layers of the seed metal catalyst ink, that is, apply only enough to induce electroless plating. If the ink lay down is too great, the ink will spread and provide wider lines and thereby reduce transparency of the article. Moreover, such thicker lines are more visible in the eventual articles and are less durable. Thus, thinner lines are desired in the patterns but this makes the electrolessly plated metal more visible in the lines through the transparent substrate.
Useful seed metal catalyst used in such materials includes metal such as particles of silver or copper. For desired properties, a sufficient amount of such metal particles can be 10% to 50% of the total weight of the ink or printable composition. At such amounts, the metal particles are generally reflective and can be readily seen through a transparent substrate, thereby adding to the visibility of the resulting electrically-conductive patterns. One attempt to reduce the reflectivity of the seed metal catalyst and electrolessly plated metals is to add sufficient colorant (such as carbon black) to the printable composition (ink) so that the seed metal catalyst is not visible. However, it is difficult to add sufficient amount of such colorants to the ink without undesirably increasing viscosity and clumping (aggregation or agglomeration) of metal particles in the ink.
Dispersants (or dispersing aids) have been commonly used to keep particulate materials in suspension as long as possible for various uses. However, it has not been possible to readily use a known dispersant with particular particles to minimize settling and particle-particle interactions that may undesirably increase viscosity of a given composition. In general, it has required considerable research and engineering in the various industries to find the best dispersants for chosen particulate materials, whether they are metallic, organic, or inorganic in nature. This has been particularly true for seed metal catalysts that are designed for electroless plating operations.
Thus, there is a need to provide a seed metal catalyst printable composition (ink) that has reduced reflectivity, small uniform particle size distribution with limited particle clumping, and suitable viscosity for application of patterns of thin lines using for example using flexographic printing.